I. Field of the Invention.
The present invention relates generally to the field of measurement and control of thrust applied to valve stems of motor-operated valves used in nuclear, petroleum, chemical and other fluid-handling and gas-handling industries.
II. Description of the Prior Art.
Since occurrence of world-alarming nuclear-power-plant disasters following decades of their use in various parts of the world, there has been a proliferation of solutions to problems with motor-operated valves that are major components of plant operation and control. In various designs of nuclear power plants, there are hundreds of motor-operated valves that are directly or indirectly critical to their operation, control and safety. As for technological development generally, advancement for the nuclear industry is advancement for other industries using the same or similar components. Thus, the many improvements in relation to motor-operated valves for the nuclear industry have been applicable in chemical and related fields, the same as intended for this invention.
The trend in improvement of motor-operated valves has been to provide means and methods for their testing in order to improve their design or to replace them if they are nonfunctional. No improvements or solutions have been made to provide full-time self-analysis, self-correction, condition-readout, automatic control and remote control of motor-operated valves in the manner taught by this invention. None have provided a sensing means for automated, remote, non-intrusive or robotic analysis and control at a seat of thrust transfer as taught by this invention.
There have been a variety of problems with motor-operated valves (MOV). Between two extremes of operating conditions, MOVs leak if they are not closed tightly enough with a gear motor and related components referred to collectively as a "valve operator" that operates them. To an opposite extreme, valve gates of various types deform their valve seats or are themselves deformed by excessive closing pressure, referred to professionally as "stem thrust". Various forms of tripping devices are employed to turn off the valve operator at a predetermined stem thrust in order to achieve design sealing characteristics without damaging either the valve components or drive components. However, testing the tripping devices, the valve components, the valve operator and related controls has required shut-down and various levels of disassembly and reassembly that are time-consuming, hazardous and expensive.
One example of an instrument for measuring valve-stem thrust is described in U.S. Pat. No. 5,257,535 granted to Evans. The Evans patent taught positioning load sensors on opposite ends of a valve housing for measuring reaction thrust or stem stress with an MOV disassembled for calibration and testing under a simulation of "actual" operating conditions. Typical of other solutions to problems with MOVs, it did not provide full-time, continuous, in-use analysis for robotic controls.
In a series of U.S. Pat. Nos. 5,000,040, 4,891,975, 4,860,596, 4,831,873, 4,759,224, 4,735,101, 4,712,071, 4,660,416 and 4,542,649, a leading authority, Charbonneau et al, taught various methods and instruments for determining operating conditions of MOVs. Some were described as "non-intrusive" by not modifying a valve operator or its circuitry for testing during in-use conditions. However, as described in the latest of this series, U.S. Pat. No. 5,000,040, these, like other instruments and methods devised in the prior art, have been related to "simulating an operation-impairing load on the valve operator". The prior art devices have been means and methods for measuring power-related parameters, for calibrating and testing instead of in-use controlling in combination with full-time analysis of the valve operator.
The Charbonneau '649 patent, for instance, describes a system which is intended to measure valve-stem thrust directly and to provide dynamic trace of the actual stem load through the valve-operating cycle. The system and related method also monitor motor current, torque and trip-switch actuation over the operating cycle. These parameters are correlated with monitored thrust. To provide an indication of actual valve-stem thrust, however, Charbonneau et al teach an apparatus with which a compression load cell is attached to a free end of a valve stem at a position opposite the valve plug. This system is limited to providing direct measurement of the stem-thrust load only at ends of valve-opening strokes. Thus it measures only thrust at which an open-torque switch trips. This single measurement of stem thrust is then used to establish both the open and closed torque-switch settings. Thus, the Charbonneau et al apparatus is incapable of monitoring and measuring stem thrust directly over the entire operating cycle.
U.S. Pat. Nos. 5,056,374 and 4,912,984 granted to McMennamy et al described load-measuring means with an overload shear for measuring thrust load on a valve stem and for eliminating the load by circumventing the valve stem under overload conditions. Neither patent taught a continuous control and monitoring device as taught by this invention. Also granted to McMennamy et al, U.S. Pat. Nos. 4,693,113 and 4,690,003 described a valve-analysis and testing system which was attached to the valve operator for statistically calibrating the valve operator relative to valve load. Further different from this invention, these McMennamy systems did not teach direct positioning of a control sensor on a stem-thrust bearing.
In U.S. Pat. Nos. 4,856,327, 5,009,101 and 5,140,853, Branam et al teach positioning of a load cell directly between a valve and a valve operator. This provides direct measurement of stem thrust in order to eliminate errors associated with indirect methods of measurement. However, load cells in this position can be rendered inaccurate and ineffective for their intended purposes by material characteristics such as heat and radioactivity, by operational parameters and by other use-related conditions of many intended applications. This is particularly true for the nuclear industry in which hundreds of valves are required to be in perfect order for handling a wide variety of materials at various temperatures. In the chemical industry also, heat of valved material under high pressure can distort current flow and readings of pressure cells that are in direct contact with materials being handled. Construction to compensate for distortions resulting from direct contact with material conditions and use conditions is expensive at best and still not reliable as a result of such a wide variation of conditions. Construction with or without compensation for such distortions also is relatively expensive. Further, the Branam et al system must be employed originally, rather than attached to existing MOVs.